<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>1px</title>
    <style type="text/css">
        .demo {
            margin: 50px;
            border-top: 1px solid #f00;
            line-height: 50px;
            position: relative;
        }
        .demo::after {
            content: "";
            width: 100%;
            height: 1px;
            transform: scaleY(0.5);
            background: #f00;
            position: absolute;
            left: 0;
            right: 0;
            bottom: 0;
        }
    </style>
</head>
<body>
    <div class="demo"> 1px 的上 border，0.5px 的下border</div>
</body>
</html>
